Have a mod idea. Don't know where to start

For a minute there I was afraid I would have to give you bad news and inform you that his mod is almost entirely script, but I checked and thankfully his is almost entirely interactions, loots, and buffs. But, to get it to apply to all sims automatically you're still going to have to do some scripting. Just thinking about the process, you might be able to get away with one script to apply your buff/trait/or something and the rest being XML.

So first thing you want to do is decide if you A) want it to be a private mod, or B) decide if you want to release it. If you decide to keep it private, the good news is you can just open his mod and add a few files + the additional script to keep things easy. But if you want to release it to the public you will have to re-make the entire mod from scratch and do it differently. I recommend keeping it private to make your life easier.

The second thing: start with something smaller than this, even if it's unrelated. Regardless of which path you choose, you'll want to learn the basics of modding first. If you jump in and immediately work on a script, or make this from scratch, etc. with zero experience you're going to probably abandon it from frustration.

Third, I don't recommend using Mod Constructor for this because you'll have to dig into the files anyhow, and MC files are barely legible. So instead stick with just Sims4Studio and Pycharm (or any Python capable program). Here are some resources:
Start with Scarlet to learn how to override and do basic tuning, she'll also cover how to setup your workspace, which programs you'll need for the most part, etc. Scarlet's Beginner tutorials (starting on page 2): www.simsvip.com/category/the-sims-4/how-to-create-a-mod/page/2/

When you've covered your basics, move on to Zer0's tutorials. They will cover how to make custom buffs, traits, interactions, and how to link them together, among some other things. Zer0's tutorials: www.patreon.com/posts/tutorials-35031545

I also have a tutorial on starting custom files: How to Start a Custom Tuning File: www.patreon.com/posts/how-to-start-38023976

Then you'll want to download some other programs/tools to help you on your journey.
You need the TDESCs and TDESC Browser as it helps you understand how files are built and what each thing does. Zer0’s tutorials linked above have 3 different tutorials on how to use the TDESC, so make sure to read them to understand how TDESCs work.
TDESCs (make sure to build your folder right, it's TDESC Folder > Subfolder for each patch TDESC): forums.thesims.com/en_US/discussion/980807/1-66-139-tdescs#latest
TDESC Browser (when referencing the folder in your settings do the main TDESC Folder): www.modthesims.info/download.php?&t=618199
When installing you need a main TDESC folder > then a subfolder for each patch's TDESC (we get new ones each patch). When choosing your folder for the TDESC Browser settings choose your main folder, not the subfolders.

XML Extractor if you want to update or make new mods before S4S updates, and XML File Finder (needs the extractor and in the same download) to see connections/references between each file without having to manually search for them. File Finder is very useful and highly recommended, even if you don’t use the XML Extractor for modding.
modthesims.info/d/563256/xml-extractor-for-the-sims-4.html

Tuning Error Notifier is to help you make sure you don’t make mistakes.
scumbumbomods.com/tuning-error-notifier/

Then for scripting we have very little documentation, but here are some links.
Beginner's Guide to Python will help you understand Python better, which is what you use to make Sims 4 scripts: wiki.python.org/moin/BeginnersGuide
Getting Started with Sims 4 Python Scripting will get your folder setup, or you can use the setup described in the tutorial series below this one: sims4studio.com/thread/15145/started-python-scripting
The Sims 4 Modern Python Modding is a new scripting tutorial series that will help teach you about Sims 4 scripting, including getting started to more difficult projects (they have 4 tutorials already and more planned as they learn themselves): Part 1
Tuning Inspector, which helps you figure out injection paths: modthesims.info/d/642704/tuning-inspector.html

And then the tutorial I think you'll need to automatically apply on aging up: sims4studio.com/thread/22036/automatically-add-traits-all-sims
It covers traits (but can be repurposed for other stuff if you need it), which you can use to setup a chain reaction from trait > to buff > that leads to loots to check on age, etc. and then apply the lock modifiers. I'm not 100% sure if that's the correct path, but it's at least an idea you can work with. There might be other paths you can take but I can't think of any off the top of my head or how to do them.

Anyhow, if you want more scripting help I would recommend joining Deaderpool's discord since not many scripters (or modders in general) journey over here, and/or for more help plus resources and tutorials there is my server Creator Musings linked in my signature.